Hydrogen sulfide, or H2S, is a clear, toxic gas with a foul odor. It is also highly flammable. The Environmental Protection Agency and other regulatory agencies worldwide strictly control the release of H2S into the environment. H2S may be present in well water, waste water and other aqueous systems. H2S is often present in crude oil and natural gas reserves and must be reduced before making commercial use of such reserves. The H2S concentration in these reserves prior to treatment typically varies with location and is usually higher in natural gas than in crude oil reserves. In natural gas reserves, for example, H2S may vary from less than 100 ppm to 3000 ppm. Permitted H2S levels will also vary by location. The U.S. limits H2S in natural gas pipelines to 4 ppm per 100 standard cubic feet (0.3 gr/100 scf).
Generally, hydrocarbon streams are treated to reduce sulfides, including organic sulfides, mercaptans, thiols, COS, and H2S, by using chemicals that will react with the sulfides. These chemicals are called scavengers, or sweetening agents. These chemical scavengers include adducts produced through the reaction of secondary amines and formaldehyde. These secondary-amine formaldehyde adduct (SAFA) scavengers include triazines, oxazolidines, Schiff bases, diamines, methyol adducts, and methylene bridge materials.
Most hydrocarbon reserves are treated continuously near the wellhead, though treating hydrocarbons in a batch or similar application elsewhere is not uncommon. Continuous treatment installations near the wellhead inject scavengers, including SAFA scavengers, directly into the hydrocarbon pipeline. The injection system typically includes a chemical injection pump and piping tees or atomization nozzles to introduce the scavengers into the pipeline. The amount of scavengers required will vary depending on a variety of factors including, the type of scavengers used, the amount of H2S in the well, permissible H2S limits, and the well flow rate. Thus, the amount of scavengers added to treat a hydrocarbon pipeline typically ranges from approximately 10 ppm to about 100,000 ppm by volume of the hydrocarbon stream. A length of the pipeline is provided to allow for contact between the scavenger and the sulfide.